Fig. 1.

Measured high-harmonic spectra of fused silica and crystalline quartz. High-order harmonics are produced by focusing intense two-cycle laser pulses onto ~100 μm thick samples of fused silica and crystalline quartz. The central wavelength of the laser is ~1700 nm (0.73 eV). A portion of the spectrum is measured, which shows that the high-energy end extends to ~25 eV. Both samples withstand repetitive excitation with the maximum peak field of ~2 VÅ⁻¹. The resulting harmonics spectra from fused silica and crystalline quartz are shown in a, b, respectively. The spectrum from fused silica consists of discrete harmonic peaks separated by twice the photon energy while the exact location of peaks depends on the carrier-envelope phase setting. The spectrum from crystalline quartz shows peaks that separate by one photon energy and merge with each other due to broad spectrum. The efficiency from crystalline quartz is higher than from fused silica. It also consists of a minimum in the range from 17–18 eV